As is known in the art of telephony, headsets are often used where the need arises for "hands-free" telephone operation. Some workers who widely utilize telephone headsets are telephone switchboard operators, travel agents, and telephone salespersons. Telephone apparatus which adapts a telephone for either handset or headset operation includes a transfer switch for selecting the desired operation and an amplifier for adjusting the volume of the voice signals transmitted to and received by the headset. Often, an amplifier module houses both the headset amplifier and the transfer switch. Such a module typically accommodates three connections: a first to the handset connector of the telephone instrument, a second to the telephone handset, and a third to the headset. In operation, when the transfer switch is in a first position, corresponding to handset operation, the module "bypasses" the telephone line signals through the telephone and to the handset to permit transmission between the service system and the handset. Alternatively, when the transfer switch is in a second position, corresponding to headset operation, signals passing between the telephone service system and the headset are amplified by the headset amplifier.
As is also known, the way in which the requisite power for operating the headset amplifier is derived has been a function of the type of telephone service system to which the telephone is connected. For example, the telephone may be connected to a Central Office system directly or alternatively, may be connected to an intermediate electronic telephone system, such as a Private Branch Exchange (PBX) or a Key system.
When a telephone is connected directly to a Central Office, ample power for operating the headset amplifier is available from the telephone. That is, when the handset is "off hook", between approximately 20 and 120 milliamps of current flows through the telephone, so that diverting the few milliamps generally necessary to power the headset amplifier does not pose a problem. However, in the case of electronic service systems, powering the headset amplifier cannot be achieved in the same manner. More particularly, when an electronic telephone goes off hook, as little as nine milliamps may flow. Thus, diverting the few milliamps necessary to power the headset amplifier is not feasible since telephone performance may be sacrificed. Conventionally, powering headset amplifiers used in conjunction with an electronic telephone service system has been achieved with an external power source, such as batteries or an AC power line.
When the handset is of one conventional type having a dynamic microphone, batteries may be a costly and inefficient way of powering the headset amplifier. That is, a dynamic microphone, having a winding and permanent magnet arrangement, does not have a sufficient signal voltage associated therewith with which to detect whether the handset is off hook and thus, whether the amplifier should be powered. If the transfer switch is in the headset position, the amplifier remains powered longer than necessary draining the batteries. While the use of an AC line to power the amplifier of a headset having a dynamic microphone is suitable, the headset user may find it objectionable to be "plugged in" to the AC wall outlet. Some headset amplifiers have the capability of operating from either battery or AC power sources, however, such does not overcome the respective disadvantages associated with either battery and AC line powering of the headset amplifier.
Headset amplifiers adapted for use interchangeably with different available handset types are typically referred to as Universal Amplifiers. Such amplifiers are manually preconfigured by the user in accordance with the particular microphone type and wiring arrangement. In the case of telephones employing carbon microphones, ample power is available at the handset connector to drive a headset amplifier. However, such is often not the case with handsets employing dynamic or electret microphones.